Missing Link in the Growth of Lead-Based Zintl Clusters: Isolation of the Dimeric Plumbaspherene [Cu4Pb22]4–

We report here the structure of an endohedral plumbaspherene, [Cu4Pb22]4–, the gold analogue of which was previously postulated to be a “missing link” in the growth of larger clusters containing three and four icosahedral subunits. The cluster contains two [Cu2Pb11]2– subunits linked through a Cu2Pb4 trigonal antiprism. Density functional theory reveals that the striking ability of mixed Pb/coinage metal Zintl clusters to oligomerize and, in the case of Au, to act as a site of nucleation for additional metal atoms, is a direct consequence of their nd10(n + 1)s0 configuration, which generates both a low-lying (n + 1)s-based LUMO and also a high-lying Pb-centered HOMO. Cluster growth and nucleation is then driven by this amphoteric character, allowing the clusters to form donor–acceptor interactions between adjacent icosahedral units or to additional metal atoms.


■ INTRODUCTION
The chemistry of Zintl ions, and in particular those containing endohedral transition metals, has seen a rapid expansion in recent years. 1−5 Much of this has been driven by an innate interest in the nature of the chemical bond in these typically highly symmetric molecules, but it is becoming increasingly apparent that they are much more than mere ornaments. Potential applications in materials science have been highlighted in the recent literature, 6 and the use of Zintl ions in catalysis is also beginning to be explored; recent examples include the catalysis of the reverse water gas shift reaction 7 and the hydrogenation of alkenes. 8 It is not always clear whether the Zintl cluster retains its structure throughout the course of the catalytic cycle, but nevertheless the presence of transition and main-group metals in a controlled ratio may play an important role in controlling reactivity. The key to realizing the full potential of Zintl ions in catalysis or in materials science will necessarily lie in the development of rational synthetic routes to generate ever larger clusters with tailored structures and elemental compositions.
Typical synthetic protocols used in contemporary work involve the combination of a main-group metal cluster (such as the Pb 9 4− precursor used in this paper) with a source of lowvalent transition metal ions, at high temperatures and in the presence of large counter-ions. The crystalline products are often highly sensitive to both air and moisture, but nevertheless a now extensive family of clusters with stoichiometries MPb 9−12 has been synthesized in this way, including [CuPb 9 ] 3− , 9 [NiPb 10 ] 2− , 10 [AgPb 11 ] 3− , 11 [MPb 12 ] n− (M = Au, 12 Ni, Pd, Pt, 13 Co, Rh, Ir, 14 Mn 15 ), and [(Cp*Ru)CuPb 11 ] 2−16 ( Figure 1). Our understanding of the mechanism of growth of these clusters from smaller component parts remains limited, although a small number of recent studies have begun to address this critical issue using a combination of X-ray crystallography, mass spectrometry, and computational analysis. 17,18 In the formation of the group-5 metal clusters [TaGe 8 As 4 ] 3− and [TaGe 8 As 6 ] 3− , for example, fusion of the known tetrahedral [Ge 2 As 2 ] 2− unit with the (unknown) Ta-containing fragments [TaGe 3 ] − and [Ta-Ge 4 As 2 ] − has been shown to provide a viable route to the isolated products. 17 The growth of even larger clusters containing multiple deltahedral units and/or transition metals, which offers the potential for a much wider range of M/E ratios along with the possibility of metal−metal bonding, presents a substantial synthetic challenge simply because the individual building blocks such as the ones shown in Figure 1 typically carry high negative charges. Nevertheless, the linking of discrete deltahedra through covalent bonds has been used to great effect in the oxidative coupling of Ge 9 4− , which can yield oligomers, polymers, and a mesoporous germanium phase. 19 A recent report has also suggested that a centered [CoGe 9 ] 5− unit can fuse to generate a condensed [Co 2 Ge 17 ] 6− cluster where two Ge 9 units share a common vertex. 20 This structural motif is in fact relatively common in Zintl-ion chemistry, 21,22 as are others where the component deltahedra are fused via edges or even hexagonal faces, 23−25 although details of their formation mechanisms remain elusive. Transition metal ions can also be used to link distinct cluster units while also buffering some of the negative charge, as for example in [Ge 9 MGe 9 ] q− (M = Cu, Zn, In), 26−28 [Ge 9 ZnGe 9 ZnGe 9 ] 10− , 29 [Ge 9 HgGe 9 HgGe 9 HgGe 9 ] 10− , 30 and polymeric [MGe 9 ] ∞ 2− (M = Zn, Hg). 31,32 In a small number of cases, metal dimer or trimer units have been used as the linker, as for example in [Pb 9 Cd-CdPb 9 ] 6− , 33 [Ge 9 M-MGe 9 ] 6− (M = Zn, Cd), 29 and [Ge 9 Au 3 Ge 9 ] 5− . 34 In a recent publication, we reported the synthesis and structures of a series of mixed Au/Pb clusters including [Au 8 Pb 33 ] 6− and [Au 12 Pb 44 ] 8− , both of which contain icosahedral Au 2 Pb 11 units arranged around a central Au 2 or Au 4 core, respectively. 11 These new structures highlight the prominent role of the centered icosahedron as a fundamental unit in cluster growth, particularly in the heavier tetrels. Given that all of the clusters in Figure 1 have been isolated, it is perhaps surprising that the coalescence or fusion of endohedral units to form larger clusters is not a more common observation. The Coulombic barrier to fusion of highly charged components probably plays a part in this, but the isolation of [Au 8 Pb 33 ] 6− and [Au 12 Pb 44 ] 8− shows that this barrier is not insurmountable. We have proposed that in these mixed Au/Pb clusters, the inter-icosahedral bonding arises from strong donor−acceptor interactions between the Pbbased HOMO of one unit and the 6s orbital of the surface Au on another. 11 Given the proven existence of clusters containing three ([Au 8 Pb 33 ] 6− and [Au 12 Pb 44 ] 8− ) icosahedral units, we also proposed that the coalescence of two such Au 2 Pb 11 icosahedra to form dimeric [Au 4 Pb 22 ] 4− might play a significant role in the initial stages of the reaction, but we were never able to isolate this or any other dimeric species, leaving a frustrating missing link between the isolated icosahedra and the condensed clusters. In this paper, we report the isolation and structural characterization of the copper analogue, [Cu 4 Pb 22 ] 4− , an observation that establishes at least that this motif is viable for mixed coinage metal/lead clusters. We then use this new information to propose an allencompassing cluster-growth pathway that is supported by detailed calculations performed with density functional theory. Crystallography. Crystallographic data were collected on a Rigaku XtalAB Pro MM007 DW diffractometer with graphite monochromated Cu Kα radiation (λ = 1.54184 Å). Structures were solved using direct methods and then refined using SHELXL-2014 and Olex2 to convergence, 36,37 where all the non-hydrogen atoms were refined anisotropically. All hydrogen atoms of organic groups were placed using geometrical considerations (CCDC reference 2054778). Full details of the crystallography are given in the Supporting Information, Table S1.
Energy Dispersive X-ray Spectroscopy. Energy Dispersive Xray spectroscopy was performed using a scanning electron microscope (Hitachi S-4800) equipped with a Bruker AXS XFlash detector 4010. Data acquisition was performed with an acceleration voltage of 20 kV and an accumulation time of 150 s.
Electrospray Ionization Mass Spectrometry. Negative ion mode ESI-MS of the DMF solutions of a single crystal of [Cu 4 Pb 22 ] 4− were measured on an LTQ linear ion trap spectrometer from Agilent Technologies, ESI-TOF-MS (6230). The spray voltage was 5.48 kV, and the capillary temperature was maintained at 300°C. The capillary voltage was 30 V. The samples were made up inside a glovebox under a nitrogen atmosphere and rapidly transferred to the spectrometer in an airtight syringe by direct infusion with a Harvard syringe pump at 0.2 mL/min.
Computational Methods. All DFT calculations were performed using the Amsterdam density functional (ADF) package, version 2019.304. 38 Slater-type basis sets of triple-zeta + polarization quality were used on all atoms, with orbitals up to 2p (Cu), 3d (Ag), and 4d (Au, Pb) included in the frozen core. 39 The Perdew−Becke− Ernzerhof (PBE) 40 functional was used in all calculations, which were spin-restricted throughout. Relativistic effects were incorporated using the zeroth-order relativistic approximation (ZORA). 41 The confining effect of the cation lattice was approximated using a continuum solvent model with a dielectric constant of 78.39. 42 Open-shell systems are computed using spin-unrestricted DFT at the same level of theory. Fragment calculations were also performed with the same functional, basis sets, and solvation model, according to the extended transition state approach of Ziegler and Rauk. 43 All stationary points were confirmed to be minima or transition states by the presence of none or one imaginary vibrational frequency, respectively. In some case, additional small imaginary frequencies (<10i cm −1 ) were found using the analytical frequencies module in ADF, but subsequent rescanning of these modes using numerical differentiation and a small step size (disrad = 0.002) showed these to be small and real.

■ RESULTS AND DISCUSSION
Patterns of Cluster Growth. There exists an already substantial body of experimental evidence in the literature that we can use as a framework to build a model of cluster growth for mixed lead/coinage metal atoms, including 1. Structural characterization of tricapped trigonal prismatic [CuPb 9 ] 3 − and (distorted) icosahedral [AuPb 12 ] 3− . 9,12 Both clusters have the skeletal electron count of 2n + 4 typically associated with a nido geometry, yet they retain the highly symmetric (albeit somewhat distorted) structures more commonly associated with a closo count of 2n + 2. The preference for these highly symmetric structures is probably driven by the spherically symmetric potential imposed by the endohedral metal, which in turn favors an approximately spherical arrangement of atoms over inherently less spherical nido alternatives. 2. Structural characterization of the approximately C 5vsymmetric nido-[AgPb 11 ] 3− cluster. 11 This also has a skeletal electron count of 2n + 4, and it does adopt a classically nido geometry with one open face. The contrast in behavior to [CuPb 9 ] 3− and [AuPb 12 ] 3− may simply reflect the fact that there is no high-symmetry structure available for an 11-vertex cluster, so the distinction between closo and nido is less sharp than in the 12-vertex analogues.

Electrospray ionization mass spectrometry (ESI-MS)
data that confirms the presence of clusters with stoichiometry MPb 11 for both M = Ag and Au, 11 M 2 Pb 9 for M = Cu, 44 11 The presence of zerovalent metal atoms in these clusters means that they can be considered as models for the earliest stages of the nucleation of Au nanoparticles.
To this body of data, we now add a new structure, that of  The ESI-MS of a solution made up by dissolving a single crystal of 1 in DMF is shown in Figure 3. A peak corresponding to the parent ion, [Cu 4 Pb 22 ] z− , is absent from the spectrum, probably reflecting the facile fragmentation of the dimer into smaller icosahedral components. The most intense peak in the spectrum is not, however, due to the fragmentation product [Cu 2 Pb 11 ] − , but rather to [CuPb 12 ] − at m/z 2550.6. If the Cu + ion is endohedrally encapsulated, this [CuPb 12 ] − cluster has a closo electron count of 50 , expands the already extensive body of data on coinagemetal clusters of Pb that has been reported in previous papers, both by us 11,12 and by other authors. 9,16 Our aim here is to collate all of the available data, both new and previously    Table  1.
Analysis of the Fundamental Steps Using DFT. Before exploring the energetics of the various steps in Figure 4, it is important to highlight the approximations and assumptions that underpin our computational model. In the synthetic chemistry described here and elsewhere, 11 the source of the coinage metal is a mesityl compound, either in the form of a cluster ((AgMes) 4 11 ) or as a phosphine complex (AuMes-(PPh 3 ) 11 or, here, CuMes(PPh 3 ) 2 ). To balance the equations in Table 1, we use the simplified model fragment MPh (Ph = phenyl) as the source of metal (the methyl groups of mesityl are removed for computational expedience) and we assume that the Ph − anion is released into solution as the metal is incorporated into the cluster. In fact it is likely that the mesityl anion abstracts a proton from the ethylenediamine solvent in the course of these reactions, 47 but the precise fate of the ligand is not critical to the arguments we make here because whatever approximations are made in modeling the ligand are, they are the same for all three coinage metals. For this reason, our emphasis throughout this discussion is on the relative energetics of the Cu/Ag/Au triad, rather than on any one specific reaction. The two columns in Figure 4 are connected by cluster expansion reactions, C, D and E that increase the Pb/M ratio while retaining the same number of coinage metal ions. As was the case for the fate of the mesityl anion, it is difficult to establish the source of additional Pb atoms in order to balance the chemical reactions: they may, for example, be extruded from the Pb 9 4− Zintl ions, or from fragments of these larger clusters, or indeed from nanoparticles of elemental Pb, which have been observed in reactions of this kind. 33 Again, this is not a significant limitation as long as our emphasis remains on the relative energetics within the Cu/Ag/Au series, where any deficiencies in the treatment of the Pb atoms in our computational model are at least constant. In the following analysis, we choose the energy of a free Pb atom in its triplet ground-state (6s 2 6p 2 ) as a convenient reference. For each step in Figure 4, three energies are given corresponding to the balanced equations for the reaction with M = Cu, Ag, and Au (the colors in the Figure correspond  offers independent support for its existence in solution. 16 We do not address here the question of how these initial endohedral fragments form, although it is likely that smaller transient components such as tetrahedral Pb 4 4− play a role, as proposed by Dehnen and Weigend in their study of Ta/Ge/As    Figure 5, and the total binding energy (ΔE total ) for the Cu + cation at the [CuPb 11 ] 3− fragment is shown in the first column of Table 2, along with its component parts according to the energy decomposition scheme proposed by Ziegler and Rauk. 43 Note that the total binding energies for M + differ from the energies for step F in Figure 4, where the reaction in question is [  Table 2, ΔE prep is the difference in energies between the two fragments in their optimized geometries and the geometries they adopt in the cluster, ΔE steric is the sum of Pauli and electrostatic energies, ΔE orbital is the energy from interaction of occupied and virtual orbitals on the two fragments (decomposed into separate contributions from the irreducible representations of the C 5v point group), and ΔΔE solvation is the difference between solvation energies of the cluster and its component fragments. The energy decomposition analysis confirms that the orbital interaction between the two fragments, [CuPb 11 ] 3− and Cu + , is dominated by the a 1 representation, and specifically the interaction of the 6a 1 orbital of the nido cluster with the empty 4s orbital of Cu + , the latter making the dominant contribution to the 7a 1 LUMO of [Cu 2 Pb 11 ] 2− . The absence of a pair of electrons in this orbital leaves the total valence electron count at 48, 2 fewer than the 4n + 2 = 50 expected for a stable closo icosahedron and renders the cluster substantially Lewis acidic. However, the doubly degenerate 5e 1    Journal of the American Chemical Society pubs.acs.org/JACS Article is non-bonding with respect to the capping atom, and its high energy confers significant Lewis basic character on the cluster. In short, the [Cu 2 Pb 11 ] 2− unit has amphoteric character: it is simultaneously Lewis acidic and Lewis basic, and this proves critical both to the subsequent dimerization step (G) and to the nucleation of additional metal atoms. It is instructive at this point also to compare the binding of the [CuPb 11 ] 3− fragment to a Cu + ion with the corresponding process with the [RuCp*] + fragment found in the stable cluster [(Cp*Ru)CuPb 11 ] 2− . 16 The frontier orbital array of [CuPb 11 ] 3− shown in Figure 5a establishes an isolobal relationship between it and the cyclopentadienyl anion, Cp − , and indeed [(Cp*Ru)CuPb 11 ] 2− can be understood as being isolobal with ruthenocene, Cp* 2 Ru. This isolobality is based on the presence of three high-lying orbitals, 6a 1 and 5e 1 , and, critically, all three participate in bonding the cluster to the [RuCp*] + fragment which, unlike a Cu + ion, does have lowlying vacant orbitals of e 1 symmetry (Supporting Information, Figure S5). The total binding energies for the Cu + and [RuCp*] + fragment shown in Table 2 Figure 6 and is fully consistent with the X-ray data summarized in Figure 2. The frontier canonical orbital domain of [Cu 2 Pb 11 ] 2− shown in Figure 5b, with a doubly degenerate HOMO and a low-lying vacant LUMO of a 1 symmetry, suggests an isolobal relationship to BH 3 which, in turn, highlights the isolobal relationship between the title cluster, [Cu 4 Pb 22 ] 4− , and diborane, B 2 H 6 (Supporting Information, Figure S7). Donor−acceptor interactions from one component of the 5e 1 HOMO of one fragment to the 7a 1 LUMO of the other stabilize the Cu 2 Pb 4 trigonal antiprism linking the two subunits. The dimerization results in a substantial re-hybridization that complicates a simple fragment-based analysis of the canonical orbitals, but orbital localization (using the Pipek-Mezey algorithm) confirms the presence of two 3-center-2-electron bonds linking the two sub-units (Figure 6b, inset). These localized orbitals resemble closely those that bind the Cu 2 Sn 4 antiprism in [Cu 2 Sn 10 Sb 6 ] 4− , 46 and indeed the total dimerization energy of −1.14 eV is very close to the value of −1.01 eV reported in that case. The very short Cu−Pb bond lengths within the trigonal antiprism (2.98 Å from DFT, Figure 6, ∼2.90 Å in the X-ray structure, Figure 2) are testament to the strength of the 3-center-2-electron bonds, as is the elongation of the Cu−Cu distances in the icosahedral units (2.54 Å in the dimer vs 2.42 Å in the isolated fragments). Ultimately, the tendency to form a dimer can be traced to the fact that the electron donor capacity of the doubly degenerate 5e 1 Figure S6) is strikingly similar to that of [Cu 2 Pb 11 ] 2− : the LUMO has dominant 4s character on the Cu + ion on the cluster surface, while the doubly degenerate HOMO is localized on the adjacent square face. The cluster is therefore also amphoteric and a dimerization step analogous to that discussed above for [Cu 4 Pb 22 ] 4− would generate a C 2h -symmetric structure where the two bicapped square antiprisms are linked via a Cu 2 E 4 trigonal antiprism, precisely the motif found in the [Cu 2 Ge 18 Mes 2 ] 4− ( Figure S8). 45,46 Instead, however, the dimerization goes a step further, to the point where the two Cu 2 Pb 9 units fuse to form a single continuous D 2h -symmetric Pb 18 cage, also shown in Figure 6. Both isomers are local minima on the potential energy surface, but the experimentally observed D 2h -symmetric structure is the more stable of the two by 0.76 eV, indicating that the driving force to coalesce to a single Pb 18 cage is substantial. Despite this, the projected density of states (PDOS) plots shown in Figure 6b reveal no significant differences in electronic structure between the two clusters: both feature well-separated maxima for the Cu 3d and Pb 6s/6p manifolds with no evidence for substantial Cu 3d-Pb covalency. The two clusters do, however, share a common icosahedral coordination geometry about the encapsulated Cu + ion: Cu@CuPb 11 in [Cu 4 Pb 22 ] 4− and Cu@Cu 2 Pb 10 in [Cu 4 Pb 18 ] 4− . It is possible, then, that the inherent stability of the icosahedron is the controlling feature, and that coalescence of two endohedral MPb x fragments, whatever their size, will proceed to the point where an icosahedral geometry is achieved. In such circumstances, it seems likely that the product distribution can only be controlled by varying the Cu/ Pb ratio in solution.
Periodic Trends in Cluster Growth: Comparison of Cu, Ag, and Au. The cluster growth pathway proposed in Figure 4 and the orbital analysis in Figure 5 are applicable to the coinage metals in general, but there are nevertheless some conspicuous differences between Cu, Ag, and Au that merit comment. The first of these is that the smallest nido-  Table 2, correlates with the gas-phase ionization energies of the metals (7.73, 7.58, and 9.23 eV for Cu, Ag, and Au, respectively 48 ) with the values for Ag being conspicuously lower than those for either Cu or Au. The relatively high energy of the 5s orbital of Ag, and the consequent weak binding of the Ag + cation to the Pb 5 face, offers an immediate explanation for the isolation of nido-[AgPb 11 ] 3− but not its Cu or Au analogues. The fact that nido-[AgPb 11 ] 3− can be isolated presents the intriguing possibility that ternary clusters of the form [AgM′Pb 11 ] 2− might be accessible through further reaction of [AgPb 11 ] 3− with a source of M′ + . The total cation binding energies for all possible combinations of M and M′ are summarized in Table 3 (note that the diagonal elements of this Table are the total energies given in Table 2). For a given M′ + , the binding The energies of the cluster expansion reactions (steps C, D, and E in Figure 4) show a rather different pattern: the reactions becoming more favorable in the order Cu < Ag ∼ Au, correlating approximately with the size of the transition metal ion (r{Cu + } = 0.97 Å, r{Ag + } = 1.29 Å, r{Au + } = 1.33 Å according to Shannon's revised tables 49 ), rather than with the ionization energies. This is intuitive: expansion of the Pb x cluster in steps C, D, and E becomes increasingly favorable as the radius of the endohedral cation becomes larger, and the clusters [MPb 9 ] 3− and [M 4 Pb 18 ] 4− are for this reason likely inaccessible for all but the smallest of the coinage metals, Cu.
The most conspicuous difference between the chemistries of Cu, Ag, and Au in these reactions is, however, the fact that we isolate larger clusters which incorporate neutral metal atoms, M, only in the case of Au. 11 The absence of equivalent clusters of Ag in the present case can probably be traced to the weak binding of Ag + to the open pentagonal face of [AgPb 11 ] 3− discussed above, which immediately rules out further condensations based on icosahedral subunits. In the subsequent analysis, we therefore focus on the comparison between Cu and Au, both of which have been shown to form clusters based on the icosahedral building block, M 2 Pb 11 . One possible explanation for the absence of larger clusters for Cu is that the new dimer reported here, [Cu 4 Pb 22 ] 4− , is a thermodynamic sink that prevents further reactions. However, the energies of step G in Figure 4 are almost independent of the identity of the metal, suggesting that the dimerization step does not effectively differentiate members of the triad.
Alternatively, the answer may lie in the more facile reduction of Au + to Au and its stronger binding to the icosahedral subunits, a topic that we explore in the following paragraphs.
The    Cu and Au, respectively): the binding of the MPh unit to the surface clearly renders the cluster more susceptible to reduction. 50 An alternative possibility, that does not involve an external reducing agent, is that the mesityl ligands may be lost via C−C coupling reactions, forming bimesityl, 51,52 for which there is precedent in the literature. 53−55 This "C−C coupling" pathway is explored in the lower half of Figure 7, and leads to the same intermediate M 6 cluster, [M 6 Pb 22 ] 4− , via the sequence (H → M → N). The reaction again involves the binding of MPh, now followed by dimerization (step M) and reductive elimination of Ph 2 (step N). It is striking that in both pathways that the step that differentiates Cu from Au most clearly is the one where the metal is reduced, either by an external reducing agent (step I) or via electron transfer from the bound phenyl ligands (step N). Indeed the difference between Cu and Au is approximately twice as large in the latter, where two Au + are reduced. Without additional experimental evidence, it is not easy to distinguish between these two pathways, but the computed activation energy for the C−C coupling reaction (step N) is 1.18 eV (50.1 kcal/mol, see Supporting Information, Figure S9), somewhat higher than the values of 31−39 kcal/mol reported by Boronat et al. 53,54 and certainly rather high for reactions that occur in the range of 40−60°C. 11 On this basis, reduction by the starting material, Pb 9 4− , with concomitant loss of Mes − , seems the more plausible route. Whichever mechanism dominates, it is clear that the process is more favorable for Au than it is for Cu due to the greater ease of reduction that is manifested in the greater exothermicity of steps I or N in Figure 7. The precise balance between dimerization to form [M 4 Pb 22 ] 4− (step G) and the formation of larger clusters will depend critically on the concentrations of the various species in solution, but the computed energy landscape suggests that nucleation of metal atoms on the surface of the cluster is more likely to prevail for Au than for Cu.

■ SUMMARY AND CONCLUSIONS
In this paper, we have reported the isolation and structural characterization of the [Cu 4 Pb 22 ] 4− cluster, the gold analogue of which was previously postulated to be the "missing link" in the growth of larger Au x Pb y clusters such as [Au 8 Pb 33 ] 6− and [Au 12 Pb 44 ] 8− . 11 The cluster itself is a dimer of [Cu 2 Pb 11 ] 2− icosahedra linked via a Cu 2 Pb 4 trigonal antiprism, the stability of which stems from strong donor−acceptor interactions between the Pb-centered HOMO of one icosahedral unit and the Cu 4s-based LUMO of the other. The system is, in fact, isolobal with B 2 H 6 , and the bonding shares much in common with this simple molecule.
The tendency of the [M 2 Pb 11 ] 2− icosahedra to dimerize or even oligomerize appears to be a general feature of the coinage metals (M = Cu, Au), which stand apart from the apparently closely-related MPb 12 systems, none of which behave in the same way. The unique ability of the coinage metal clusters to dimerize and oligomerize is a direct consequence of their nd 10 (n + 1)s 0 configuration: the vacant (n + 1)s orbital confers a high degree of Lewis acidity, while the absence of vacant nd orbitals leaves the Pb-based HOMO high in energy and available to act as a Lewis base. This amphoteric character also allows the clusters to act as a nucleation site for additional zerovalent metal atoms, which leads to the agglomeration of larger clusters [Au 8 Pb 33 ] 6− and [Au 12 Pb 44 ] 8− . The 5s orbital of Ag is higher in energy than either the 4s of Cu or the 6s of gold, and as a result, the chemistry of Ag stands out as quite distinct in that the only isolated product is nido-[AgPb 11 ] 3− , where a second Ag + cation does not bind at the open face. The contrasting Zintl cluster chemistry of the Cu/Ag/Au triad is, therefore, an elegant illustration of the alternation of periodic properties commonly encountered in this region of the periodic table.
■ ASSOCIATED CONTENT
Black details of crystal data and structural refinement, asymmetric unit of the crystal and views of the unit cell, EDX spectrum and analysis. The computational analysis is expanded to include Kohn-Sham MO diagrams, optimized structures of isomers, a discussion of isolobal relationship, the transition state for C-C coupling, formation energies of metal atoms and cations, and a summary of total formation energies of all clusters and fragments (PDF)